ports: Allow build scripts install to custom path

This will maybe allow packaging ports :)

ports/dbus/build.sh

@@ -25,5 +25,5 @@ build() {
 }
 
 install() {
-	meson install --destdir="$BANAN_SYSROOT" -C build || exit 1
+	meson install --destdir="$DESTDIR" -C build || exit 1
 }